On nearly every distributary there are some watercourses whose command is bad, and it has been stated ([Chapter II, Art. 9]) that in an ordinary unmoduled distributary the sizes of the outlets in such cases should be extremely liberal. To module any such outlet would cause a lowering of the water level in the watercourse and would interfere with the irrigation. Such outlets should not be moduled. Again, there are some few outlets which are not submerged, i.e., there is a free fall into the watercourse. The discharge does not depend on the water level in the watercourse, and it is not affected by any enlargement or clearance of it. It depends only on the water level in the distributary. This water level, if most of the outlets are moduled, will be fairly constant. Such outlets need not be moduled, and they should not be moduled unless the other unmoduled outlets in the reach concerned are sufficiently numerous, and perhaps not even then, because moduling involves some expense.
A distributary generally has some falls which divide it into reaches. Immediately upstream of a fall the water level for a given discharge is not affected by the silting or scouring of the channel. Any outlets near to and upstream of the fall are less subject than others to variation in discharge, and are suitable for non-moduling in case a sufficient number of unmoduled outlets is not otherwise obtainable.
Regarding the watercourses at the extreme tail of a distributary it has been pointed out ([Chapter III., Art. 7]) that in an ordinary case they should not be left without masonry outlets, because they may then lower the water level and so unfairly reduce the supply of any watercourse, even though upstream of them, which has such an outlet. But any outlets near the tail of a distributary can suitably be left unmoduled because of the difficulty of ensuring that the supply at the tail shall be exactly what is needed.
Gibb’s modules have been tried on various distributaries in the Punjab and found to give good results. It is believed however that in only one case has a whole distributary been moduled. The distributary is a large one, its length being 35 miles. It appears that the discharge reaching the tail of the distributary is not constant but varies, as was to be expected, when the head discharge varies for any length of time. The command on the distributary is good. There is nothing to show that matters would not have been improved, and money saved, by leaving some of the outlets without modules.
It has been remarked above, that at the downstream end of a reach ending in a fall, the F.S. level of a distributary is not affected by silt. At the upstream end of the reach it is affected. There are thus two gradients, one flat, and one steep. It appears to have been decided in one case in the Punjab, that the minimum limit of supply for the module should be about half an inch below the flat line and the maximum limit ·3 feet above the steep line. In many cases a greater range would be required,[55] say a foot.
[55] It is understood that a range of a foot can easily be arranged for, and that ranges of 3 or 4 feet can be introduced at slightly increased cost.
In [Chapter III. Art. 7], the case of a distributary without modules but with the outlets carefully adjusted, was considered. The question to be decided in each case is whether such an arrangement is preferable to moduling some of the outlets. This turns largely on the amount of attention which would be bestowed on the case. In view of the difficulty of securing such attention and of the trouble of constantly making alterations in a certain number of outlets, it is probable that moduling will in many cases be considered preferable.
The question of moduling the heads of distributaries has also been considered in the Punjab. For minor or small distributaries modules are feasible. For a large distributary a module would be expensive and it appears that the present system of regulating is preferable.
Kennedy’s “Gauge Outlet,” which is a kind of semi-module is described in [Appendix K]. It is being tried in the Punjab.